The production of gasoline motor fuel requires consideration of the balance between the specifications provided by the automobile manufacturers and the concern for the environment as controlled by the governmental regulations on automobile emissions. Renewed environmental awareness and the desire for cleaner air on the part of the public has encouraged gasoline producers to develop reformulated grades of gasoline to reduce emissions from automobiles. Government has supported this reformulation initiative with new regulations which will result in the addition of oxygenates such as alcohols and ethers to the gasoline pool in an effort to reduce the level of CO and hydrocarbon emissions compared to emissions from conventional gasoline grades. The reformulated grades of gasoline, often referred to as oxyfuels, must meet all the typical gasoline specifications, and in addition must contain a minimum amount of oxygen. In the United States, according to current regulations, this oxyfuel must be sold in those areas of the country which do not meet minimum standards for ozone pollution.
Automotive gasoline is usually sold by a grade such as regular, or premium, according to its octane rating. This octane rating is a measurable quality and is derived from a laboratory measurement of octane number. The octane number is a rating of the performance of a sample of the gasoline in a standard test engine. Typically, two types of octane numbers are used to characterize the octane rating (i.e., a research octane (RON) and a motor octane (MON). These are determined separately according to well-known laboratory methods and averaged (RON+MON)/2 to provide an octane rating for a particular grade of gasoline.
Oxygen may be added to gasoline in the form of an oxygenate such as an alcohol including methanol, ethanol, or isopropanol and the like, or an ether including methyl tert.-butyl ether (MTBE), ethyl tert.-butyl ether (ETBE), tert. amyl-methyl ether (TAME), and the like. Oxygenates are added to the gasoline pool comprising hydrocarbons in amounts such that the octane rating and oxygen content of the blend increases, without exceeding vapor pressure limits. Vapor pressure is a physical property which reflects the amount of volatile material in the motor fuel. A high vapor pressure can result in hydrocarbon emissions to the atmosphere. Although alcohols such as methanol and ethanol have favorable octane numbers when blended with other gasoline components, these alcohols generally have a higher vapor pressure than ethers. Therefore, the gasoline producers have sought to increase the oxygen content of fuels by incorporating more renewable resource materials such as ethanol into the gasoline by converting the alcohols into ethers by combining the alcohols with C.sub.4 and C.sub.5 iso-olefins, or isoalkenes, over an acid catalyst.
The production of ethers by the reaction of an iso-olefin and an alcohol is a wellknown commercial operation. A number of detailed descriptions of such processes, particularly as they relate to the production of methyl tert.-butyl ether (MTBE) appear in the technical and patent literature. Exemplary of patent disclosures are U.S. Pat. No. 3,726,942 issued Apr. 10, 1973, to K. E. Louder; U.S. Pat. No. 4,219,678 issued Aug. 26, 1980, to I. Obenaus et al; U.S. Pat. No. 4,447,653 and U.S. Pat. No. 4,575,567 issued to B. V. Vora on May 8, 1984, and Mar. 11, 1986, respectively; and U.S. Pat. No. 4,876,394 issued to M. M. Nagji et al Oct. 24, 1989. These ethers are useful as high octane blending agents for gasoline motor fuels by virtue of their high Research Octane Number (RON) of about 120 and their low volatility.
MTBE has become the most commonly used ether for gasoline octane improvement. For example, a typical reformulated gasoline grade would require about 11 volume % MTBE to provide a gasoline containing about 2.0 wt % oxygen before reaching a vapor pressure limit. In a similar manner, if ETBE were used, the resulting blend with about 2.7 wt % oxygen would accommodate about 17 volume % ETBE at the same vapor pressure limit. ETBE has a higher octane value than MTBE and a blending vapor pressure of about one-half that of MTBE. In addition, ETBE like MTBE is miscible in gasoline in all proportions, but ETBE has a lower water solubility than MTBE, giving ETBE better fungibility in gasoline blends. ETBE is less likely than MTBE to be lost in pipeline transport.
The cost of production is a major factor on the use of MTBE over ETBE. Methanol is typically derived from natural gas, while ethanol is generally produced by fermentation of organic material. Given appropriate favorable price equalization of ethanol relative to methanol, the goal of encouraging the use of more regenerable material in the gasoline pool may be achieved. ETBE is produced by an etherification reaction of ethanol and an iso-olefin, such as isobutylene, wherein ethanol is present in an amount in excess of that required for the reaction. Typically, the reactor effluent is fractionated to produce a light stream comprising unreacted hydrocarbons and an ETBE product stream. Although some of the excess ethanol will be withdrawn with the unreacted hydrocarbon stream, at least a portion of the ethanol generally will remain in the ETBE product. The ethanol remaining in the ETBE product results in a loss of ethanol, and this ethanol significantly raises the vapor pressure and lowers the octane rating of the ETBE product. European Patent No. 542596 discloses the use of a costly and energy intensive extraction and three-stage fractionation scheme to separate the unconverted ethanol from the ETBE. Methods are sought to perform the separation of the ETBE from ethanol in the ETBE product in an efficient and low cost manner, without the loss of any valuable gasoline blending components.
A problem with the removal of ethanol from the ETBE by an adsorptive separation process is that most adsorbents that are capable of selectively adsorbing ethanol from ETBE to produce an ether product essentially free of ethanol also adsorb other polar compounds such as water and tertiary butyl alcohol. When water is present in the feed or recycle stream to a reactor for the production of ethyl tert.-alkyl ether, the water will react with the isoalkene in the feedstream to form a tertiary alcohol. When the isoalkene is isobutylene, tert. butanol, or tert. butyl alcohol (TBA) is formed. When the isoalkene is isoamylene, tert. amyl alcohol (TAA) is formed. If the TBA is adsorbed along with the ethanol in the adsorptive separation process, the TBA will be returned to the reaction zone and eventually build up in the reaction zone. Water generally forms an azeotrope with ethanol making it uneconomical to separate water from the ethanol recycle stream, and the cost of separating TBA from the unreacted ethanol stream by conventional means would be prohibitively expensive. Thus, there is no way to remove TBA from the recycle stream once it is produced. Although some control of TBA production is provided by maintaining the water level in the feedstream at very low levels, this approach does not protect the reaction zone from water introduced with the ethanol recycle stream. As TBA builds up in the reaction zone, the temperature of the reaction zone is generally raised to maintain conversion. When the concentration of TBA exceeds moderate levels in the combined feed, the higher temperature required to maintain adequate conversion adversely affects catalyst stability resulting in poor yields of ether and premature catalyst failure. In reaction with distillation systems for the production of ethyl tert. butyl ether, where the reaction zone is contained within a fractionation column, the TBA collects in the bottom of the column. This increased TBA concentration in the bottom of the column requires the operating temperature of the column to be raised to maintain vapor traffic in the column, to maintain the degree of conversion in the reaction zone, and to obtain the degree of separation of the finished product. The combination of these factors significantly increases operating the cost and reduces efficiency of producing the ether.
Processes employing adsorptive separation of ethanol from the ether product are sought which minimize or prevent the build-up of tertiary alcohol in the reaction zones while producing an ethyl tert.-alkyl ether essentially free of ethanol.